Low pressure mercury vapour discharge lamp



Dec. 1, 1970 AKIRA SQMEYA ET AL 3,544,829

LOW PRESSURE'MERCURY VAPOUR DISCHARGE LAMP Filed Feb. 5. 1969 FIG! United States Patent 3,544,829 LOW PRESSURE MERCURY VAPOUR DISCHARGE LAMP Akira Someya, Yokohama-shi, Teizo Hanada, Saitamaken, and Tadaaki Watanabe, Hyogo-ken, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-shi, Japan, a corporation of Japan Filed Feb. 3, 1969, Ser. No. 796,031 Claims priority, application Japan, Feb. 3, 1968, 43/6,428 Int. Cl. H01k 1/56 US. Cl. 313-178 10 Claims ABSTRACT OF THE DISCLOSURE A low pressure mercury vapour discharge lamp has a getter disposed on said mounts except those portions thereof which are coated with said activated electronemitting materials. The getter comprises a mixture of a least one metal selected from a first group consisting of nickel, cobalt, iron, aluminium and copper, and at least one metal selected from a second group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, scandium, cerium and tungsten, said mixture containing 5 to 90 percent by weight of said at least one metal selected from said second group. Said getter functions to decrease the occurrence of end bands while said discrage lamp is lit, with no increase in anode spots.

The present invention relates to improvements in a low pressure mercury vapour discharge lamp, and more particularly to a. low pressure mercury vapour discharge lamp, wherein there is employed a getter consisting of a mixture so as to decrease the occurrence of end bands and prevent the increased generation of anode spots.

Low pressure mercury vapour discharge lamps include ordinary fluorescent lamps for general lighting purpose, black light lamps, sterilizing lamps, erythemal fluorescent lamps, etc. These are all discharge lamps operable by the low pressure mercury vapour.

With the above-mentioned low pressure mercury vapour discharge lamp, there appear blackening phenomena at the end of the sealed enevolpe. These are classified into end bands and anode spots. The occurrence of the former results from the fact that the mercury oxide formed by reaction between mercury and the oxygen gas released from the electron-emitting material during operation of the lamp is deposited on the inner wall of the sealed envelope facing the Faradays dark space having a small electric potential gradient. The appearance of the latter is caused by the sputterings of the electron-emitting material deposited on the inner wall of the sealed envelope facing the electrodes.

The aforementioned blackening phenomena spoil the visual appeal of a low pressure mercury vapour discharge lamp and reduce the effective amounts of light beams. To eliminate these drawbacks, the present inventors previously proposed to utilize as getters various alloys to adsorb the evolved oxygen gas thereby to reduce the formation of end bands and prevent the increased appearance of anode spots. The inventors further experiments, however, show that a mixture whose composition is properly controlled is also available for use as a getter. Such mixed getter has the advantage of being more easily prepared than the alloy getter.

It is accordingly the object of the present invention to provide a low pressure mercury vapour discharge lamp wherein the getter consists of a mixture to adsorb the oxygen gas evolved from the electron-emitting material during the operation of the lamp thereby to minimize the 3,544,829 Patented Dec. 1, 1970 ice occurrence of end bands and prevent the increased formation of anode spots.

According to the present invention, there is provided a low pressure mercury discharge lamp comprising a lighttransmissible sealed envelope, a quantity of mercury and a starting rare gas sealed in said envelope, and a pair of electrode mounts sealed to both ends of said enevolpe, said electrode mounts each supporting a filament coated with activated electron-emitting materials, characterized by comprising a getter disposed on said mounts except those portions thereof which are coated with the activated electron-emitting materials, said getter including a mixture of at least one metal selected from a first group consisting of nickel, cobalt, iron, aluminium and copper, and at least one metal selected from a second group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, scandium, cerium and and tungsten, said mixture containing 5 to percent by weight of said at least one metal selected from said second group.

The feature of the present invention is that in a low pressure mercury vapour discharge lamp, there is employed a getter comprising a mixture of at least one metal selected from a first group consisting of nickel, cobalt, iron, aluminium and copper and at least one metal selected from a second group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, scandium, cerium and tungsten, the proportion of the metal of the second group varying between 5 and 90 percent by weight on the basis of the entire mixture, said mixture being disposed on the electrode mounts except those portions thereof which are coated with electron-emitting materials.

The present invention can be more fully understood from, the following detailed description when taken in connection with the accompanying drawing, in which:

FIG. 1 is a cross section of part at a low pressure mercury vapour discharge lamp according to the present invention, with part of the envelope broken away to show an electrode mount;

FIG. 2 is a perspective view of the flared stem in another embodiment of the invention; and

FIG. 3 is a perspective view of the flared stern used in a further embodiment of the invention.

The low pressure mercury vapour discharge lamp shown in FIG. 1 is a straight tube type fluorescent lamp having a rated wattage of 20 watts. It consists of a cylindrical glass envelope 1, the inner walls of which are coated with fluorescent materials and a pair of electrode mounts 2 (only one of them is shown) sealed to both ends of said envelope. The electrode mount 2 is formed of a flared glass stem 3 sealed airtight to the envelope 1 and a pair of leadin wires 5 and 6 penetrating said stem 3, the inner ends of which constitute the inner-lead wires. To the innerlead wires is welded a filament 7, which is coated with activated electron-emitting materials such as BaO-SrO-CaO containing MgZrO To the outside of the stem 3 is fitted by proper cement 11 a base shell 8 having a pair of base pins 9 and 10. The aforementioned lead-in wires 5 and 6 are connected to the pins 9 and 10 respectively. On the suitable portion of the surface of the inner-lead wires are disposed getters 12 and 13 respectively. As hereinbel ow described with respect to the preferred examples, the getter is prepared from at least one metal selected from a first group consisting of nickel, cobalt, iron, aluminium and copper and at least one metal selected from a second group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, scandium, cerium and tungsten mixed in such a manner that the content of the latter metal accounts for 5 to 90 percent by weight on the basis of the entire mixture. The getter is prepared and disposed in the following way. The metals of the first and second groups are separately pulverized to an average particle size of 100 microns or less. Those metal powders are fully mixed in specified proportions using a binder consisting of, for example, nitrocellulose. The mixture is suspended in a solvent consisting of, for example, butyl acetate. The suspension is applied on the suitable portion of the inner-lead wires. Thus the getters 12 and 13 are easily disposed. Since the solvent is dried off and the binder is completely expelled during the operation of evacuating the lamp envelope, the getter is in no way obstructed in performing a gettering action.

FIG. 2 shows another type of eelctrode amount used in a low pressure mercury vapour discharge lamp according to the present invention. The electrode mount consists of a flared glass stem 20, a pair of lead-in wires 21 and 22 connected to the base pins of a base shell (not shown) through the stem 20, a filament 23 stretched across the lead-in wires 21 and 22 and having its both ends clamped by the inner ends of said lead-in wires respectively and wire anodes 24 and 25 are attached to the lead-in wires respectively. The wire anodes 24 and 25 are coated with getters 26 and 27 respectively.

Still another type of electrode mount of FIG. 3 com prises a flared stem 30, a pair of lead-in wires 31 and 32 and a filament 33. In this modification, a shield electrode 34 is provided in a manner to surround the filament 33. The shield electrode 34 is fixed by a support pin 35 embedded in the stem 30. The outer surface of the shield electrode 34 is coated with a getter 36. It will be understood that the positions where the getter is to be disposed are not limited to those described in the foregoing embodiments, but may be shifted, for example, to the surface of the flared stem 3, 20 or 30. 1

The composition of the mixed getter consisting of at least one metal of each of the first and second groups is presented by way of illustration in the table below.

Composition in parts Time of continuous Example by weight and average lighting and index of N 0. particle size and bands produced 1 {Ti (10 microns) 60 parts. 10in 3,000 hrs.

""""""""" gi ((10 microns; parts. 10 to 9 in 6,000 hrs.

i 10 microns parts 2 "{W ((11%microns)) 58%partts to 9 m 20001115 Ni microns par 5 3 l'%h((l0 microns)) 20 parts to 9 m 2000 1115' r 10 microns 50 parts 4 "int (10 microns) 50 parts 10 3:900 1115' 5 {N1 (10 microns) 50 parts 10 to m 2,000 hrs.

""""""""" 11 a ((110 microns; 50 parts 8 in 6,000 hrs.

e 0 microns 50 parts. 6 "{Ti 10 microns) 50 parts 8 6100 The mixed getters represented by the above examples 1 to 6 were coated with 20 mg. of metal materials along (namely, exclusive nitrocellulose and butyl acetate) for each lamp. The amounts of electron-emitting material used in each lamp were 10 mg. for all examples. The index of end bands shown in the table above represents the extent of their occurrence as computed on the ten-mark basis. Namely, the index of 10 marks denotes no formation of end bands, the index of 7 marks means that the presence of end bands is distinctly observed, and that of marks or less indicates that the lamp presents an unattractive appearance due to the prominent formation of end bands. In the case of Example No. 4, the sealing of an electrode mount to the envelope had to be carried out while the mount was cooled in streams of nitrogen, because if the portion of the mount which was to be coated with a getter had an elevated temperature, the getter mixture would be scattered due to combustion.

Examples Nos. 1, 2, 3, 5, and 6 did not exhibit any formation of anode spots, whereas Example No. 4 displayed a tendency toward the appearance of anode spots.

By way of comparison, there was prepared a lamp coated with 20 mg. of titanium powders microns in average particle size and 10 mg. of electron-emitting material. During 100-hour operation, the lamp presented a prominent occurrence of anode spots, though the ind x of end bands stood at 10- Th re w s. also p pared a reference lamp by applying 10 mg. of electron-emitting material, but without using any getter mixture. The lamp indicated an end band index of 8 during the initial 2000- hour operation, said index being reduced to 5 to 4 during the subsequent 6000 hours.

As clearly seen from these comparison, a low pressure mercury vapour discharge lamp comprising a getter mixture specified by the present invention is remarkably improved with respect to the generation of end bands, though it is not much different from the prior art discharge lamp regarding the effect of reducing the formation of anode spots.

Referring to the technical effect of the present invention there may be made the following assumption. The metal of the aforementioned first group has high electric conductivity, and that of the second group is very active and capable of acting as a getter. As mentioned above, where the getter only consists of titanium, a metal of the second group, there prominently occur amode spots. This is also the case with other metals of the second group. The metals of the second group have extremely strong chemical activity, so that during the evacuating operation involved in the manufacture of a low pressure mercury vapour discharge lamp, the metal absorbs the gases released from a flourescent material, as well as those evolved when the carbonates contained in the electronemitting materials are thermally decomposed. Accordingly, when the lamp is finished, said metal already reaches a saturated condition in respect of adsorption, and thus, said metal becomes unuseful condition for reducing the end band. Moreover, the strong reducing power of the second group metal causes the barium contained in the electron-emitting material to be freed in excess amounts during the lamp operation, which also accelerates the formation of anode spots. In contrast, where as in the present invention, the getter consists of the metal of the first group having good electric conductivity mixed with that of the second group having strong chemical activity, then the gettering action of the second group metal, namely, the oxygen adsorbing action in this case, is suitably restricted by the presence of powders of the first group metal, preventing the second group metal from being saturated during the evacuating operation. Further, the oxygen adsorbed to the second group metal gradually diffuses into the interior of the getter mixture due to the presence of the powdered first group metal, enabling the surface of the getter again to restore its adsorbing power. After all, the mixed getter according to the present invention is supposed to be effective for prevention of the formation of end bands, because it always has a suitable power to adsorb the free oxygen. The fact that the powdered second group metal is surrounded by the powdered first group metal of high electric conductivity restricts temperature rise in the anode cycle, which in turn prevents the cathode spots from unduly increasing in temperature in the cathode cycle. This is deemed to be a factor in controlling the generation of anode spots.

Where the proportion of the second group metal falls below 5 percent by weight on the basis of the entire mixture, the resultant getter mixture will undesirably display substantially no oxygen-adsorbing action and not be effective to prevent the occurrence of end bands. If the content of said group metal exceeds 90 percent by weight on the same basis it will also be obectionable due to the remarkable appearance of anode spots. thought it may be effective to suppress the formation of end bands. For the purpose of the present invention, therefore, the proportion of the second group metal should be limited to the range of from 5 to 90 percent by weight. And where said proportion stands at between 20 and percent by weight there will appear particularly prominently the effect of suppressing the formation of end bands.

The getters of the aforementioned examples are formed by one metal of the'first group mixed with one metal of the second group. However, it'may be possible to select two or more metals from either or both of said groups in preparing a mixed getter.

What is claimed is:

1. A low pressure mercury vapour discharge lamp comprising a light-transmissible sealed envelope, a quantity of mercury and a starting rare gas sealed in said envelope, and a pair of electrode mounts sealed to both ends of said envelope, said electrode mounts each having a pair of lead-in wires through a stem and supporting a filament coated with activated electron-emitting materials, characterized by comprising a getter disposed on said mounts except those portions thereof which are coated with the activated electron-emiting materials, said getter including a mixture of at least one metal selected from a first group consisting of nickel, cobalt, iron, aluminum and copper, and at least one metal selected from a second group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, scandiurn, cerium and tungsten, said mixture containing to 90 percent be weight of said at least one metal selected from said second group.

2. A low pressure mercury vapour discharge lamp according to claim 1, wherein said mixture consists of 60 percent by weight of titanium and 40 percent by weight of nickel.

3. A low pressure mercury vapour discharge lamp according to claim 1, wherein said mixture consists of 50 percent by weight of nickel and 50 percent by Weight of tungsten.

4. A low pressure mercury vapour discharge lamp ac cording to claim 1, wherein said mixture consists of 80 percent by weight of nickel and percent by weight of thorium.

'5. A low pressure mercury vapour discharge lamp according to claim 1, wherein said mixture consists of percent by weight of nickel and 50 percent by weight of tantalum.

6. A low pressure mercury vapour discharge lamp according to claim 1, wherein said mixture consists of 50 percent by weight of iron and 50 percent by weight of titanium.

7. A low pressure mercury vapour discharge lamp according to claim 1, wherein said mixture contains 20 to percent by weight of said at least one metal selected from said second group.

8. A low pressure mercury vapour discharge lamp according to claim 1, wherein said lead-in wires are disposed with said getter, on the portions thereof extending from said stem.

9. A low pressure mercury vapour discharge lamp according to claim 1, wherein said electrode mounts each is provided with a pair of anode wires on the surface of which said getter is disposed.

10. A low pressure mercury vapour discharge lamp according to claim 1, wherein said electrode mounts each is provided with a shield electrode on the outer periphery of which said getter is disposed.

References Cited UNITED STATES PATENTS 2/1951 Peters 313109 7/1951 Homer 313178 RAYMOND F. HOSSFIELD, Primary Examiner US. Cl. X.R. 313-109, 174 

